The present invention relates to load-responsive braking pressure control devices in general, and more particularly to a device of this type which is especially suited for use in vehicle brake systems.
In conventional devices of this type, a control valve is actuatable by a stepped piston which is slidable in a housing. The control valve is urged in its closing direction by the outlet pressure which acts on a larger effective surface of the stepped piston and in its opening direction by the inlet pressure which acts on a step of the stepped piston which has a relatively smaller effective surface. A load-responsive spring, in particular a suspension spring supporting the vehicle body on the axle, acts on the stepped piston in the opening direction through the intermediary of a transmitting device.
In a load-responsive braking pressure control device of this type which is disclosed in the U.S. Pat. No. 4,150,855, a suspension spring designed as a compression spring bears against the vehicle body, on the one hand, and against a spring seat element, on the other hand. The spring seat element surrounds the housing for the stepped piston. The spring seat element acts on several radially extending levers. The outer ends of these levers are seated on a vehicle unsprung mass, and their inner ends engage a cap. The levers compress a preloading spring through a pin, and the spring loads the stepped piston. The provision of such levers is necessary in this construction to ensure that only part of the vehicle spring force is transmitted to the stepped piston. The transmitting device resulting therefrom is complicated from the structural and manufacturing points of view. Difficulties are involved in designing the lever ratios such that only a small amount of the force of the suspension spring is transmitted to the stepped piston; this has to be taken into account when choosing the size of the stepped piston. Moreover, the stepped piston is under constant preload. The stepped piston is thus required to be dimensioned in conformity with such continuous load. The stepped piston moves only upon attainment of the changeover pressure defined by the preload. To ensure timely closing of the control valve, the piston stroke until the instant of closing is permitted to be but short. This results in tolerance problems during the manufacture and assembly and in difficulties during the operation.